Optically sensing devices such as charge coupled device (CCD) are widely used in image readers. Referring to FIG. 1, an inputted image signal 11 is broadened by the optical sensing device 12, which will trouble the recognition of the waveform when converting the inputted analog data into a binary digital signal.
There are two conventional methods for converting an analog signal from an image reader to a binary digital signal. The first method is to take a second differentiation of the waveform of the analog signal to obtain the edge of the waveform. Because a differentiation to a waveform is analogic to a high-pass filter to an analog signal, which will be coupled with a lot of disturbance signals, and thus decrease the accuracy in reading image data.
Another method is to take a voltage value, normally a mean value of the maximum and the minimum voltage of the analog signal, as a threshold voltage. The point having a higher voltage value than the threshold voltage is considered to have a high level voltage in the binary digital signal, while the lower voltage value point is considered as a low level voltage. This method is invalid in a high resolution requiring job, thus an optical character recognition (OCR) process will fail without a high image quality.